1. Field of the Invention
The invention relates to a method and a device for detecting and measuring the foetal heart rate (FHR) during labour and more specifically for detecting and measuring the decelerations in the heart rate during the uterine contractions during labour.
2. Description of the Prior Art
It is now known that there is a correlation between the foetal heart rate and the foetal state and accordingly the state of the child to be, as indicated in the article of G. Sturbois, M. Tournaire, A. Ripoche, R. Le Houezec, G. Breart, J. Chavine and C. Sureau published in the "Journal of Perinatal Medecine", 1-1973, PP 235-244 and entitled "Evaluation of the foetal state by automatic analysis of the heart rate".
The prior art will be explained with reference to FIG. 1a which shows as an example part of a foetal electrocardiogram (EKG). It will be appreciated that in this EKG, the QRS complex is clearly distinguishable from the background noise. This complex corresponds, with a possible phase shift, to the heart beat. As shown in FIG. 1b, the complex QRS, and more specifically the wave R, can easily be shaped by a known electronic device for providing a pulse train. The time distance between two of these pulses defines an elementary period of the EKG. And the variation of this period is in correlation with the foetal state. In an ideal case, this period is substantially regular and constant. When this period increases, that is, the time interval between two successive heart beats increases, there is a deceleration. When this time interval decreases, that is when the heart rate increases, a case of tachycardia exists. However, a direct interpretation of an EKG as shown in FIG. 1 is practically impossible. Indeed, it can be indicated, for example, that a normal FHR is of about 120 pulses/minute, i.e. the elementary period is of about 500 milliseconds. When this period increases to about 600 milliseconds, a deceleration occurs. But such an increase of the period is, in practice, not easily distinguishable from the curves shown in FIGS. 1a and 1b. However, it is necessary to be able to analysis the EKG and the possible deceleration very rapidly if one wants to help sufficiently the practitioner who must be able to make a quick decision in the event of those decelerations being repeated and reaching an "alarm threshold," decision which can be for example to intervene in the delivery of the foetus, for example by carrying out a cesarian operation or by the use of forceps.
Consequently, an electronic monitoring apparatus of the foetal heart rate (FHR) during labour which provides, from the electrical signal shown in FIG. 1b, a curve, namely "period curve," the x axis of which represents the time and the y axis of which represents the time interval between a pulse and the immediately preceding pulse has been provided in the prior art. If the heart rate has a constant value, this curve is a horizontal line, but if a deceleration occurs, the y value of the curve increases. However, this period curve is complex and with difficulty interpreted by those non-specialized in the art. Moreover, this interpretation may vary from one observer to another. The use of such an apparatus by the practitioner would require special training as well as experience.
This curve is particularly difficult to interpret as it has to be considered in relationship with the changes in the intra-uterine pressure due to the contractions. Furthermore, recent studies have shown that, in the period curve, the quantitative values had to be carefully analysed. And from the period curve, three characterizing values appear as presenting clear coalitions with the foetal state. Those characteristics are: (1) the "linear" measurement of the decelerations, that is the sum of the differences between the deceleration values and the average value; (2) the total area of deceleration, that is the product of the deceleration amplitude into the deceleration duration; and (3) the simultaneous and residual deceleration surfaces, that is the deceleration surfaces measured during the contractions and the deceleration surfaces measured between the contractions, respectively. The hereabove theoretical study has been made by comparing the measurements obtained from the period curves processed by a computer, making a statistical study of a great number of cases, as it is shown in particular in the above-cited article. However, it is clear that such a processing by a computer cannot be carried out practically in each maternity center. Another important difficulty uncovered during research made with the help of a computer is that the average value of the foetal rate changes during labour, which makes it very difficult to define the deceleration threshold.